Surface polysaccharides of Gram-negative enteric pathogens are both essential virulence factors and protective antigens. Enteric infections and their extraintestinal complications continue to be a major cause of morbidity and mortality throughout the world especially in children. The immunogenicity, and therefore the potential for efficacy, was enhanced by binding Vi polysaccharide of Salmonella typhi to the recombinant Pseudomonas aeruginosa exoprotein A (rEPA) by two synthetic schemes using the heterobifunctional linker SPDP and adipic acid dihydrazide (ADH). Phase 1 studies confirmed the superior immunogenicity of both Vi conjugates over Vi alone. The Vi-rEPA, prepared with ADH was significantly more immunogenic than Vi conjugates prepared with SPDP. Sequential Phase 2 studies in Vietnam, an area with a high attack rate of typhoid and other enteric fevers, showed that both Vi conjugates elicited booster responses in 2-4 year olds and that the ADH-linked conjugate was significantly more immunogenic than the SPDP-conjugate. In preparation for a Phase 3 study in children, conjugates of Salmonella paratyphi A, the second most common cause of enteric fever in Southeast Asia, were found to be safe and immunogenic in adults, teenagers and then 2-4 year old children. Treatment with acetic acid anhydride of a plant polysaccharide, pectin, produced an antigen with identical antigenicity as Vi. A di-O-acetylated pectin conjugate, prepared for a Phase 1 study in Vietnam, is be compared with Vi conjugates. Escherichia coli O157, an emerging pathogen, causes hemolytic uremic syndrome in young children. Phase 1 study of O157 O-specific polysaccharide-rEPA elicited greater than or equal to 4-fold rise of anti-IgG anti-LPS with bactericidal activity in all adult volunteers after four weeks. The B-subunit of Shigella toxin II was bound to the O157 O-specific polysaccharide. The bivalent conjugate elicited bactericidal antibodies to E. coli O157 and neutralizing antibodies to Shigella toxin II. Additional clinical studies with E. coli O157 conjugates for active and passive immunization are planned.